In general, electrochemical cells have a mono polar structure. Such a mono polar electrochemical cell includes a positive electrode composed of a positive active material formed on a current collector and a negative electrode composed of a negative active material formed on another current collector. These electrodes are disposed with opposite polarity sides facing each other, and a separator is inserted between the electrodes to form a unit-cell structure.
FIG. 1 illustrates a mono polar electrochemical cell of the related art.
Referring to FIG. 1, the electrochemical cell 10 of the related art includes a positive electrode 11, a negative electrode 12, a separator 13, an electrolyte 14, terminals 15-1 and 15-2, and a case 16. The illustrated electrochemical cell is the minimum basic operation unit which is called a unit cell.
Electric energy is stored in the positive electrode 11 and the negative electrode 12.
The separator 13 inserted between the positive and negative electrodes 11 and 12 is electrically nonconductive. However, the separator 13 may be omitted if the positive and negative electrodes 11 and 12 can be spaced apart from each other without the separator 13. In a recent battery such as a lithium polymer battery, solid polymer electrolyte is used instead of a separator; however, the solid polymer electrolyte contains a liquid electrolyte, and electrochemical reactions are produced by ions contained in the liquid electrolyte. That is, basically, the lithium battery is not different from a battery using a separator and a liquid electrolyte.
The separator 13 is formed of a material capable of transmitting the electrolyte 14, such as porous polymer, fiber glass mat, and paper.
The operational voltage of such electrochemical unit cells having the above-described structure is only several volts. Among electrochemical cell batteries, a lithium ion battery has a relatively high operational voltage; however, the nominal voltage of the unit cells of the lithium ion battery is also low at about 3.6 Volts.
Therefore, as shown by unit cells 21, 22, and 23 in FIG. 2, electrochemical cells should be connected in series for being used in application fields such as industrial and vehicle application fields requiring several tens to several hundreds of volts.
Since the unit cells 21, 22, and 23 are connected in series, the assembled structure and assembling processes are complicated, and additional parts such as bus bars and screws are necessary. Furthermore, the volume, weight, and resistance of the assembled structure are increased. As shown in FIG. 2, bus bars are used to connect neighboring unit cells, and screws are used to fix the bus bars to the unit cells.
An electrochemical cell 30 having a bipolar structure as shown in FIG. 3 has been developed to address the above-described limitation.
In the electrochemical cell 30 having a bipolar structure, electrodes having opposite polarities are formed on both sides of current collectors 31 and electrodes having opposite polarities face each other with a separator 32 being disposed therebetween. The lowermost electrode is composed of an active material layer formed on one side of the lowermost current collector 31, and the uppermost electrode is composed of an active material layer formed on one side of the uppermost current collector 31.
In manufacturing electrodes of the bipolar electrochemical cell 30, if positive and negative electrodes are formed on the same material of the current collector 31, a positive active material layer 33 and a negative active material layer 34 are formed on both sides of the current collector 31 having a sheet shape. If positive and negative electrodes have to be formed on different materials of the current collector 31, a complex current collector having a laminated structure formed of different materials is used as the current collector 31. In FIG. 3, reference numeral 35 denotes gaskets, and reference numerals 36 and 37 denote terminals. The gaskets 35 are used as electrolyte sealing and isolating members for sealing unit cells, such that undesired phenomena such as current leakage, side reactions, corrosion caused by the side reactions can be prevented between unit cells.
Generally, in a lithium ion battery, a current collector used for a positive electrode is formed of aluminum, and a current collector used for a negative electrode is formed of copper. In a lithium ion battery having a bipolar structure, current collectors having a multi-layer structure composed of aluminum and copper lamination sheets may be used. In a general electrochemical cell having a bipolar structure, an electrolyte isolation member is installed on an edge portion of an electrode so as to prevent undesired phenomena between unit cells, such as current leakage, side reactions, and corrosion caused by the side reactions. For the same reason, an electrolyte should not be transmitted through a current collector of an electrode in the electrochemical cell having a bipolar structure.
In the bipolar structure, if electrolytes of neighboring unit cells are not securely isolated, current leakage occurs between the unit cells, and the unit cells corrode easily. Therefore, it is very difficult to isolate electrolytes of neighboring unit cells securely for a long time under various operation environments.
Another limitation of a bipolar electrochemical cell is that it is difficult to manufacture a high-capacity bipolar electrochemical cell. The areas of electrodes should be increased to increase the capacity of a bipolar electrochemical cell; however, in this case, the structural strength of the bipolar electrochemical cell is reduced, and it is more difficult to isolate electrolytes of neighboring unit cells and inject electrolyte into the unit cells. Furthermore, it is troublesome to assemble electrodes and separators into an electrochemical cell after electrolyte is filled between the electrodes and the separators.
An electrochemical cell having a quasi-bipolar structure similar to the bipolar structure has been developed.
FIG. 4 is a cross-sectional view illustrating a quasi-bipolar electrochemical cell of the related art.
Referring to FIG. 4, the quasi-bipolar electrochemical cell 40 includes current collectors 41, separators 42, positive active material layers 43, negative active material layers 44 and 45, and gaskets 46.
FIG. 5 is a perspective view illustrating an electrode of an electrochemical cell having a quasi-bipolar structure according to the related art. In the above-described bipolar electrochemical cell, active material layers having opposite polarities are disposed on both sides of a current collector. However, as shown in FIG. 5, a quasi-bipolar electrochemical cell 50 includes mono polar electrodes and a quasi-bipolar electrode. The mono polar electrodes include current collectors 51 and 52, and positive and negative active material layers 53 and 54 respectively disposed on the current collectors 51 and 52 for being connected to terminals. The quasi-bipolar electrode includes a current collector 56, and positive and negative active material layers 57 and 58 disposed on the current collector 56 and spaced apart from each other with a current collector extension part 55 being located therebetween.
The electrodes are arranged in a manner such that electrodes having opposite polarities face each other, and separators are disposed between the electrodes. In the quasi-bipolar structure, the quasi-bipolar electrode is used as opposite electrodes of neighboring unit cells. That is, neighboring unit cells are connected in series to each other through the current collector extension part of the quasi-bipolar electrode. In a bipolar structure, a current flows in a direction perpendicular to electrodes; however, in a quasi-bipolar structure, a current flows in a direction parallel to electrodes, that is, in a direction parallel to current collectors. In a quasi-bipolar electrochemical cell, an electrolyte isolation member, such as a gasket and an adhesive that are formed of a nonconductive material through which electrolyte cannot be transmitted, is disposed on a current collector extension part of a quasi-bipolar electrode located at the center portion of the quasi-bipolar electrode so as to isolate electrolytes of neighboring unit cells. However, if there is no extra electrolyte except for electrolyte at an active material layer of an electrode and a separator, such an electrolyte isolation member is not always necessary. In a sealed recombination lead acid battery, extra electrolyte does not exist at other regions than an active material layer of an electrode and a separator, and although extra electrolyte may exist, the extra electrolyte evaporates by an electrochemical reaction. Therefore, in a certain case, an electrolyte isolation member may be not necessary.
In manufacturing electrodes of an electrochemical cell having a quasi-bipolar structure, if the same material of a current collector is used for positive and negative electrodes, electrodes are formed by a generally used active material forming method using a sheet, mesh, or grid current collector; however, if different current collector materials are used for positive and negative electrodes, after positive and negative electrodes are formed in a manner such that an active material does not exist at edge portions of a current collector, the portions where an active material does not exist may be electrically connected by an electric connecting method such as welding so as to form electrodes. Generally, the surface of a current collector is treated like an etched aluminum foil to increase the surface area of the current collector so as to attach an active material layer to the surface of the current collector more reliably.
In a method of forming a high-capacity quasi-bipolar electrochemical cell, unit cells are formed by stacking a plurality of electrodes. According to the method, a positive or negative electrode is formed on a portion of a side of a current collector, and another electrode having an opposite polarity is formed on the other portion of the side of the current collector. Then, positive and negative electrodes are formed on the other side of the current collector in a manner such that electrodes having the same polarity overlap each other with the current collector being disposed therebetween. At this time, current collector extension parts where no active material exists are formed on both sides of the current collector between the positive and negative electrodes. Thereafter, the electrodes are connected in series to each other in a manner such that one polarity of an electrode is used as an opposite polarity in a neighboring cell. That is, as shown in FIG. 6, a plurality of electrodes are stacked in a manner such that electrodes having opposite polarities face each other with a separator being disposed therebetween. As shown in FIG. 6, an electrochemical cell 60 having a stacked quasi-bipolar structure includes negative active material layers 61 and 62, separators 63, positive active material layers 64, current collectors 65, and gaskets 66. Gaskets or an adhesive made of an electrically nonconductive material impermeable to electrolyte may be disposed at current collector extension parts and between electrodes and a case, so as to isolate electrolyte of unit cells from neighboring unit cells.
U.S. Pat. No. 3,167,456 discloses a structure in which both sides of an electrode are supported by a spacer used as an electrolyte isolation member instead of using a separator for supporting the electrode. U.S. Pat. Nos. 3,941,615 and 4,734,977 disclose structures in which an electrolyte isolation member and a separator are used. U.S. Pat. Nos. 4,504,556 and 4,964,878 disclose structures in which an electrolyte isolation member is not used between unit cells.
In such a stacking type quasi-bipolar structure, an electrolyte isolation member may not be used between unit cells, or electrically nonconductive electrolyte isolation member may be used between unit cells. Therefore, electrodes having the same polarity and stacked in a unit cell are not connected in parallel. Voltage variations in such a stacking type electrochemical cell having a quasi-bipolar will now be described with reference to FIG. 7.
FIG. 7 is a view for explaining voltage variations of a stacking type electrochemical cell having a quasi-bipolar structure in the related art. In FIG. 7, reference numerals 71, 72, 73, and 74 denote current collectors, an electrolyte isolation barrier wall, active material layers, and separators, respectively. The capacitance of one of the active material layers is 2C+Δ, and the capacitance of the others is 2C.
Referring to FIG. 7, electrodes disposed in a nonconductive barrier wall are stacked in two layers to form a two-series stacking type quasi-bipolar structure. If it is assumed that the electrochemical cell is an electric double layer capacitor for simplifying calculations, voltage variations of the electric double layer capacitor from a discharged state to a charged state may be calculated by Equations below.
                              V          1                =                              2            ⁢                          (                                                4                  ⁢                  C                                +                Δ                            )                        ⁢                          I              t                                            C            ⁡                          (                                                16                  ⁢                  C                                +                                  7                  ⁢                  Δ                                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]                                          V          2                =                              4            ⁢                          (                                                2                  ⁢                  C                                +                Δ                            )                        ⁢                          I              t                                            C            ⁡                          (                                                16                  ⁢                  C                                +                                  7                  ⁢                  Δ                                            )                                                          [                  Equation          ⁢                                          ⁢          2                ]                                          V          3                =                                            (                                                8                  ⁢                  C                                +                                  3                  ⁢                  Δ                                            )                        ⁢                          I              t                                            C            ⁡                          (                                                16                  ⁢                  C                                +                                  7                  ⁢                  Δ                                            )                                                          [                  Equation          ⁢                                          ⁢          3                ]                                          V          4                =                                            (                                                8                  ⁢                  C                                +                                  3                  ⁢                  Δ                                            )                        ⁢                          I              t                                            C            ⁡                          (                                                16                  ⁢                  C                                +                                  7                  ⁢                  Δ                                            )                                                          [                  Equation          ⁢                                          ⁢          4                ]            
As shown by Equations 1 to 4, if electrodes disposed in the barrier wall have different capacitances, voltage deviation occurs although the electrodes have the same polarity. In addition, if some of the electrodes stacked in the barrier wall are short-circuited, voltage deviation may occur. As described above, one of the most dominant limitations of a bipolar or quasi-bipolar structure is current leakage between unit cells caused by an electrode bridge between the unit cells. Furthermore, in the case of a stacking type quasi-bipolar structure, voltages of electrodes having the same polarity and disposed in a barrier wall can deviate due to current leakage caused by a partial electrolyte bridge. Such voltage deviation affects the lifespan and reliability of an electrochemical cell. That is, voltage equalization is necessary for the bipolar or quasi-bipolar structure due to its structural weakness. However, in the case of the stacking type quasi-bipolar structure, conductors should be connected to all electrodes disposed in each barrier wall for voltage equalization. This results in complicated structure and assembling.